1. Field of the Invention
Unsaturated polyester molding compounds have been known and widely used in the industry for several decades. The origin of sheet molding compound (SMC) and bulk molding compound (BMC), which is an unsaturated polyester resin thickened, generally chemically, has a more recent beginning. Credit seems to be given to Vincent Frilette and Charles F. Fish for the development of chemical thickeners that would permit the manufacture of reinforced and unreinforced molding compositions comprising an unsaturated polyester resin and a vinyl monomer which could be mixed, handled, and stored prior to molding. This development gave wide latitude to compounders and molders of unsaturated polyester resin. It now became possible to compound large batches of a molding composition and store it in a form such as in sheet or in bulk for use at a later time. Prior to this development, it was difficult to obtain tack free compounds and to avoid resin rich molding. Resin rich molding occurs with nonthickened compounds in that the polyester resin separates from the glass.
In the manufacture of sheet molding compound or bulk molding compound, the chemical thickener, promotor, catalyst, reinforcing agent, fillers, and lubricants are blended with the unsaturated polyester resin component and the unsaturated or vinyl monomer component. Subsequently, the molding compound is formed into a sheet and wound onto a reel or it is formed into pellets or sausages in bulk.
It is an objective in SMC and BMC manufacture to find promotors and catalysts which can be blended in the molding composition at the time of compounding that will permit the compound to remain substantially inactive at temperatures below the molding temperature to give suitable "shelf life" and yet be sufficiently reactive for catalyzing or promoting the cure at the molding temperature. Many of the so called catalysts or promotors for curing unsaturated polyester resin are unsuited for the manufacture of sheet molding compound or bulk molding compound because they are so reactive that they cause gelation at room temperature within minutes or a few days.
2. Description of the Prior Art
A number of patents report the use of metallic salts in promoting the cure rate of unsaturated polyester resin. Typical soluble salts of reactive metals include manganese octanoate, cobalt naphthenate, cobalt decanoate, lithium octanoate, lithium thiocyanate, aluminum laurate, aluminum octoate and gallium octoate. These salts, however, are too reactive and result in SMC and BMC having a short shelf life.
U.S. Pat. No. 3,988,290 contributed substantially to the state of the art and discloses the use of cobaltic salts as promotors for unsaturated polyester resins chemically thickened with a Group II metal oxide or hydroxide. Typically, cobaltic acetylacetonate is included with magnesium oxide or magnesium hydroxide for the purpose of extending the shelf life of the unsaturated polyester resin, and for providing an enhanced rate of cure at molding temperatures.
U.S. Pat. No. 3,584,076 discloses a method for initiating polymerization of an unsaturated polyester resin by incorporating a peroxide or perester catalyst, e.g. t-butyl perbenzoate in combination with a cobalt salt, e.g. cobalt octoate and an enolizable ketone such as 2,4-pentanedione. However, the molding compound has a short shelf life compared to other systems.
U.S. Pat. No. 3,398,213 discloses the use of a chelating agent in combination with a cobalt salt and peroxide catalyst as an initiator for unsaturated polyester resins. Cobalt II acetylacetonate and cobalt naphthenate were some of the cobalt salts suggested as being suited for initiating polymerization.
U.S. Pat. No. 3,239,581 discloses a method for accelerating the polymerization of an unsaturated polyester by including small catalytic amounts of a cobalt compound in combination with a free radical forming catalyst, e.g. a peroxide, or perester. Additionally, a trivalent phosphorus compound is added to the composition to prevent discoloration of the unsaturated polyester resin.